Exposure to light is the key mechanism that enables a proper synchronization of the body clock with the solar day cycle. Timing, duration, intensity and spectral composition of light exposure all have impact on the so-called entrainment of a person to a 24-hour circadian rhythm. It has been shown that restorative sleep can only occur in synchronicity with the body clock. For certain people who have a phase shift of their internal body clock relative to the social schedules around them, exposure to bright light at well-defined times can be used to shift their body clock forward or backwards to better align it with their social needs. Also for the treatment of seasonal affective disorder, timed and regular exposure to bright light is an effective means.
The assessment of light levels to which a person is exposed over the course of multiple days or even weeks is an important instrument for the diagnosis of mood and sleep disorders. Existing actigraphy products measure the activity and light exposure with a body-worn device, for example, a wrist-worn device that is used and worn similar to a wristwatch.
Such a wrist-worn device is shown, for example, in US 2008/0319354 A1, showing a system and method for monitoring information related to sleep. The wrist-worn device shown in this document comprises an illumination sensor to provide an information related to the intensity of ambient illumination of the user. The signal of the sensor can be further processed by suitable electronic computing means.
One of the problems to be solved with the present light sensing devices is the accurate measuring of the light the subject is exposed to in his daily life. The challenge is to achieve an acceptable level of accuracy given the profusion of different light sources (natural light, artificial light, light emitting devices (LED), halogen, neon etc.) with distinct spectrum contributions and intensities varying over several orders of magnitudes. This is difficult to be obtained for all conditions, and the common devices are typically pre-calibrated for a specific range of illuminations.
In general, devices including light sensors are calibrated by the manufacturer before being sold. Moreover, the original calibration may have been done inaccurately or may have been realized for different conditions, not corresponding to the intended use of the costumer (in situations with different light intensities or sources). As mentioned above, a wide range of light sources is met in daily life with very distinct spectra and intensities, and achieving a uniform accuracy level under these conditions has been turned out to be almost impossible. In addition, the light sensor characteristics may be altered over time due to temperature differences, or inadequate power supply, for example, when the battery integrated into the device gives a lower voltage. All these factors contribute to the general observation that, when deployed in the field, the light measurements are often of poor accuracy.